When assembling or disassembling pipe strings which are lowered into or down to a well in the subsurface, for example for production of petroleum, so-called power tongs or torque tongs are normally used for connecting or disconnecting pipe lengths to or from the pipe string. A common configuration is where an upper pipe part is manoeuvred and held by a gripping device, while a pipe part located below is rotated by means of a rotatable torque tong. Another configuration is where the pipe string is held by a first power tong and/or slips in the drill floor, while a rotatable torque tong rotates a pipe element above with the necessary torque. The rotatable torque tong is provided with gripping jaws in order to hold the pipe in the centre of the torque tong, so that when the torque tong rotates, the pipe element will also rotate about its longitudinal axis. In other words, the torque tong's axis of rotation and the pipe element's longitudinal axis are substantially coincident.
Developments in the field of well drilling, such as in connection with directional drilling, have created the need for higher torque and larger angles of rotation, e.g. 60°-90°. More powerful drilling machines (e.g. so-called top drives) which subject the pipes to greater torques than before, result in a need for rotary tongs which can handle correspondingly higher torques during connection (“make-up”) and disconnection (“break-out”).
Many of the known torque tongs are capable of performing a rotation of between 30°-45°. If it is necessary to rotate the pipe further when using these torque tongs, the pipe has to be released from the gripping jaws, the torque tong rotated back to the initial position and the rotation repeated. This operation may have to be repeated two or three times, which is time-consuming and increases the risk of errors and damage.
The known torque tongs have hydraulic cylinders which rotate the tong directly. This produces different forces, depending on whether the cylinder pushes or pulls, and this can make it difficult to achieve an accurate control of the torque and may give an unbalanced power picture. With rotation over large angles it is difficult to control geometry and thereby the torque. The torque is usually calculated by measuring the hydraulic pressure in the cylinders, together with the cylinders' stroke length (which is measured by means of sensors). With torque tongs of this type it is also necessary to have sensors in order to demonstrate the cylinders' end stroke, in order to prevent the end stroke from being confused with torque build-up. Known solutions require a radial support. This gives increased friction, resulting in a loss of torque for the tong which is difficult to demonstrate or measure its extent. In some cases the friction may also vary with the torque.
There is a need for a tong which can rotate over larger angles than hitherto known, thereby making it possible to screw the pipes together with sufficient torque in one operation, without having to take a new grip. There is furthermore a need for a rotatable tong which provides a constant force, without the need for sensors or the like in order to calculate torques or forces as is the case in the prior art.